The present invention relates to a method for the preparation of a wire-formed silicon crystal having a diameter of 1 mm or smaller or, in particular cases, 0.8 mm or smaller or, in more particular cases, 0.5 mm or smaller which is useful as a wave guide for light having a wavelength in the infrared region as well as in the application fields such as low-cost solar cells, strain gauges, micromachines and the like.
As is well known, silicon as a semiconductor material is widely used as a principal material in the electronic and optoelectronic devices and the importance thereof is increasing year by year as a key material supporting the information-predominant society in the present and future days. Semiconductor materials of silicon currently under practical applications in the manufacture of electronic devices is almost exclusively a single crystal in the form of a wafer which is prepared in a process in which polycrystalline silicon having an extremely high purity is used as a starting material from which a single crystal rod of silicon having accurately controlled quality is grown by the so-called Czochralski (CZ) process or floating zone-melting (FZ) process and the thus obtained single crystal rod of silicon is sliced perpendicularly to the growing axis. Since the productivity of semiconductor devices from such a silicon wafer can be improved by the use of a silicon wafer having an increased diameter, an outstanding trend in the CZ or FZ process is to produce a single crystal rod of silicon having a larger and larger diameter to be at a stage of practical production of a single crystal rod of 8-inches diameter and single crystal rods of silicon having a diameter of 12 inches or even larger are now under development.
The largest problem in the above described conventional process is that the single crystal rod of semiconductor silicon can be produced only at a very high cost and, in addition, the step of slicing of the single crystal rod to prepare wafers is unavoidably accompanied by a large material loss due to the slicing margins resulting in an increase in the overall production costs of semiconductor silicon wafers as a bottleneck when cost reduction is required for various semiconductor devices.
Along with the trend in recent years toward higher and higher performance and function required in electronic devices, on the other hand, the devices must have a finer and finer structure. A technological approach to meet these requirements is utilization of the semiconductor silicon material in the form of a thin film to constitute one of the major currents in the material technology. The progress in the technology for the preparation of thin-film semiconductor silicon serves to the development of important application fields in the direction for the availability of larger and larger liquid crystal display panels and solar cells. When consideration is made for the optical applications of semiconductor silicon or for the above mentioned problem of the kerf loss in the preparation of wafers by slicing, it would be a promising way to have a silicon semiconductor material in the form of a thin wire or filament. Nevertheless, the technological efforts for the manufacture of wire-formed materials are heretofore limited to oxides and halides only and no reports are available on the preparation of wire-formed silicon single crystals. These situations have driven the inventors to conduct extensive investigations for the development of a preparation method of a wire-formed silicon crystal.